I have found wikipedia useful on the basics, especially around form factor, chemistry, and protection circuitry (or not) in lithium. for example, 18650 lithium cell now popular in flashlights, computers and cars that can be very confusing when it is referenced with protection circuitry and 18650 does not have protection officially.
Also funny to see "and lithiums are 3V" (not mentioning li-ion at all) next to... the 3.7V lions they're selling on the same page.
Fortunately the actual page on li-ion gives 3.6V
Personally I think the safety risks of using li-ion cells like 18650s are a bit overexaggerated -- the torch/"vape" community has been using them in large quantities (in China they are becoming nearly as common as AAs), and there have only been a few notable cases of significant damage. Yes they are not as safe as something like an alkaline but even lead-acid and NiMH/NiCd can source significant amounts of current too.
These days lMR(IMR) batteries (so called safe chemistry) are probably most commonly used by "vape" community.
The benefit is that they do not explode and proven to be safe choice when you need high drain batteries. Granted, you need a good charger for them, to make sure you're not overcharging them, but this should be on your safety list anyway.
Hah, how nice it's here today. I just started a small hardware project (hopefully a Show HN in a month or two) and I got hit by the fact that I don't understand jack squat about batteries, especially Li-Ions and Li-Polys.
Important thing I learned today: 9 Volt batteries are crap. I was just about to re-build my charcoal grill air blower which used two 9V's. Now I will go with some AA's or even a lantern battery.
9V's aren't crap, they're useful because you can linearly regulate them down to the majority of the common bus voltages used in electronics. Mainly they're by far the simplest way to get 5V in a portable system (9V + LM7805). LiPOs will do fine for anything below 3.7V so most modern systems with 3.3/2.7/1.8 will be OK. But if you need a 5V system with anything aside from a dedicated supply or a 9V battery you need to include a step-up converter which adds complexity (though some ICs only need a few capacitors) and cost.
The main downside [for a 9V battery] is that linear regulation is inherently wasteful and you lose (4V x your current) to heat. They also have a comparably low capacity compared to AA's so they're not the best choice for a long life application. In that case you're better off using a 95%+ efficient switch mode supply, but then if your system requires low noise you have other considerations.
9v+7805 is fine if you're playing around, but if you're making something to use and especially if it's to be powered continuously, you'd be better off with 4 AAAs and a low-quiescent low-dropout regulator. 9v batteries can't supply much current (a guideline of 20mA is mentioned in the article). The first 7805 datasheet I came across specifies it's bias current at 4.3mA typ/8mA max. So just by powering the regulator, you're drawing a significant load from the battery even if the rest of the circuit is doing nothing.
Funnily enough, I've never seen a stacked 9v as pictured in the article. The few that I've opened up have had elongated cells laid out like a six pack of beer.
Well quite, 4xAAA is a better choice in lots of cases. Just a point that it's still a very common design pattern, particularly in cheap devices. When you look at the capacities, 9V batteries are awful.
Bear in mind that 9V batteries also have far easier interfaces than AAA packs, requiring a single plastic/metal tab rather than an injection moulded retainer. In some cases, the price difference for both manufacture and battery cost makes it worthwhile.
As for current, plenty of devices run under 20mA happily. A microcontroller in active mode will probably use under 15. Most 9V devices aren't used continuously, my IR temp gun uses a single 9V for instance. However I've also seen smoke alarms and other home sensors that use them.
I'm not disagreeing with any of the criticism here, I would never use a 9V batt in a project, but there is good reason to consider it and people should understand why so many products still use them.
Stacked 9V batteries are typically the old Alkaline sort or rechargable.
In the name of strengthening the case for 9V batteries:
Not many people know this, but it's now possible to get lithium, secondary (aka "rechargeable") batteries. They really put out about 8.0V, but sub swell in many apps.
And have ~70mA constant discharge rate.
I've been enjoying using them in many applications, esp with a buck-switching regulator.
Fair enough. What I meant to say is that they suck in terms of power density. They are obviously a useful form factor, but having a battery that lasts for a while would be better, especially in my application.
These guys (http://blueshiftpdx.com) have a speaker system that runs entirely on super caps. It charges in 5 minutes and plays for 6 hours. Downside I guess is its a bit expensive.
